System for measuring periodic structures

ABSTRACT

A periodic structure is illuminated by polychromatic electromagnetic radiation. Radiation from the structure is collected and divided into two rays having different polarization states. The two rays are detected from which one or more parameters of the periodic structure may be derived. In another embodiment, when the periodic structure is illuminated by a poly chromatic electromagnetic radiation, the collected radiation from the structure is passed through a polarization element having a polarization plane. The element and the polychromatic beam are controlled so that the polarization plane of the element are at two or more different orientations with respect to the plane of incidence of the polychromatic beam. Radiation that has passed through the element is detected when the plane of polarization is at the two or more positions so that one or more parameters of the periodic structure may be derived from the detected signals. At least one of the orientations of the plane of polarization is substantially stationary when the detection takes place. To have as small a footprint as possible, one employs an optical device that includes a first element directing a polychromatic beam of electromagnetic radiation to the structure and a second optical element collecting radiation from the structure where the two elements form an integral unit or are attached together to form an integrated unit. To reduce the footprint, the measurement instrument and the wafer are both moved. In one embodiment, both the apparatus and the wafer undergo translational motion transverse to each other. In a different arrangement, one of the two motions is translational and the other is rotational. Any one of the above-described embodiments may be included in an integrated processing and detection apparatus which also includes a processing system processing the sample, where the processing system is responsive to the output of any one of the above embodiments for adjusting a processing parameter.

BACKGROUND OF THE INVENTION

[0001] This invention relates in general to optical techniques formeasuring periodic structures, and in particular to an improvedspectroscopic diffraction-based metrology system for measuring periodicstructures such as grating-type targets on semiconductor wafers.

[0002] In conventional techniques, optical microscopes have been usedfor measuring the critical dimension (“CD”) for semiconductorlithographic processes. However, as the CD becomes smaller and smaller,it cannot be resolved at any practical available optical wavelengths inoptical microscopy. Scanning electron microscope technology hasextremely high resolution. However, this technology inherently requireslarge capital expenditures and heavy accessory equipment such as vacuumequipment, which makes it impractical for integration with lithographicprocesses. Two-theta scatterometers face similar practical challengesfor integration, as they require mechanical scanning over a wide rangeof angles. For this reason, they are slow and difficult to integratewith process equipment.

[0003] Ellipsometric techniques have also been used for CD measurements.In U.S. Pat. No. 5,739,909, for example, Blayo et al. describe a methodof spectroscopic ellipsometry adapted to measure the width of featuresin periodic structures. While ellipsometric methods may be useful forsome applications, such methods have not been able to measurereflectances from the periodic structures.

[0004] None of the above-described techniques is entirely satisfactory.It is therefore desirable to provide an improved technique for measuringperiodic structures where the above-described difficulties arealleviated.

SUMMARY OF THE INVENTION

[0005] In many of the diffraction-based metrology or ellipsometrictechniques for measuring the critical dimension, theoretical modelsemploying libraries or non-linear regression are employed to find thecritical dimension or other parameters of the periodic structure. Wherethe periodic structure measured is topographically complex, it may benecessary to measure more than one radiation parameter to get adequateinformation for the modeling process. Where the periodic structure issimple, however, the measurement of a single radiation parameter may beadequate. The use of fewer radiation parameters in the modeling processreduces the calculation time required so that the critical dimension andother parameters of the structure can be found quickly. The embodimentsof this invention are simple in construction and flexible and may beused for measuring one or more radiation parameters from the radiationdiffracted by the periodic structure.

[0006] In one embodiment of the invention, when the structure isilluminated by polychromatic electromagnetic radiation, radiation fromthe structure is collected and divided into two rays having differentpolarization states. The two rays are detected, from which one or moreparameters of the periodic structure may be derived. In anotherembodiment, when the periodic structure is illuminated by polychromaticelectromagnetic radiation, the collected radiation from the structure ispassed through a polarization element having a polarization plane. Theelement and the polychromatic beam are controlled so that thepolarization planes of the element are at two or more differentorientations with respect to the plane of incidence of the polychromaticbeam. Radiation that has passed through the element is detected when theplanes of polarization are at two or more positions so that one or moreparameters of the periodic structure may be derived from the detectedsignals. At least one of the orientations of the plane of polarizationis substantially stationary when the detection takes place.

[0007] When a device for measuring periodic structure parameters isemployed in a production environment, such as when it is integrated withwafer processing equipment, it is desirable for the device to have assmall a footprint as possible. One embodiment of the invention adaptedfor such environments employs an optical device that includes a firstelement directing a polychromatic beam of electromagnetic radiation tothe structure and a second optical element collecting radiation from thestructure where the two elements form an integral unit or are attachedtogether to form an integrated unit.

[0008] One way to reduce the footprint of the apparatus for measuringthe periodic structure in a wafer processing environment is to move theapparatus relative to the wafer without moving the wafer itself.However, since the apparatus includes a number of components, it alsohas a significant size and footprint compared to that of the wafer.Furthermore, it may be cumbersome to control the sizable apparatus sothat it can move in a two dimensional plane without moving the wafer.Thus, it is envisioned that both the apparatus for measuring theperiodic structure and the sample (e.g. wafer) are caused to move. Inone embodiment, translational motion of the apparatus and the wafer iscaused where the two motions are transverse to each other. In adifferent arrangement, one of the two motions is translational and theother is rotational. This facilitates the handling of the motion of theapparatus while at the same time reduces the overall footprint.

[0009] Any one of the above-described embodiments may be included in anintegrated processing and detection apparatus which also includes aprocessing system processing the sample, where the processing system isresponsive to the output of any one of the above embodiments foradjusting a processing parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1A is a schematic view of an apparatus for measuring one ormore parameters of the periodic structure to illustrate one embodimentof the invention.

[0011]FIG. 1B is a schematic diagram of the plane of incidence and theplanes of polarization of the polarizer and analyzer of FIG. 1A toillustrate the operation of the apparatus of FIG. 1A.

[0012]FIG. 1C is a schematic diagram of another apparatus for measuringone or more parameters of the periodic structure to illustrate analternative embodiment of the invention particularly suitable in anintegrated processing and detection tool to illustrate anotherembodiment of the invention.

[0013]FIG. 2 is a schematic diagram of yet another measurement devicefor measuring one or more parameters of the periodic structure toillustrate yet another embodiment of the invention.

[0014]FIG. 3 is a schematic diagram of illumination and collectingoptics employed in the different embodiments of this invention toillustrate an asymmetric numerical aperture for illumination andcollection.

[0015]FIG. 4A is the perspective view of a semiconductor wafer and anapparatus for measuring periodic structures on the wafer and instrumentsfor moving the apparatus and the wafer to illustrate yet anotherembodiment of the invention.

[0016]FIG. 4B is a perspective view similar to that of FIG. 4A exceptthat the apparatus of FIG. 4B is self-contained in the optical head andincludes a polychromatic electromagnetic radiation source.

[0017]FIG. 4C is a schematic view of a semiconductor wafer and of opticsarranged so that the optical path length remains substantially constantirrespective of the positions of the moving optics head relative to thewafer and illustrates yet another embodiment of the invention.

[0018]FIG. 5 is a schematic view of a system for moving the optics headalong a straight line and for rotating the wafer.

[0019]FIG. 6 is a schematic diagram of an apparatus illustrating in moredetail the components in the optics head in FIGS. 4A, 4C.

[0020]FIG. 7 is a schematic view of an apparatus for measuring periodicstructures illustrating in more detail an alternative construction ofthe optics head in FIGS. 4A, 4C.

[0021]FIG. 8 is a schematic view of the different components in theoptics head to illustrate in more detail the optics head of FIGS. 4B, 5.

[0022]FIG. 9 is a flowchart illustrating the operation of theapparatuses of FIGS. 1A, 1C and 2.

[0023]FIG. 10 is a schematic block diagram illustrating a waferprocessing apparatus including a track/stepper and an etcher and aspectroscopic measurement device where information from a periodicstructure and/or associated structures from the device as used tocontrol the manufacturing process and the track, stepper and/or etcherto illustrate the invention.

[0024]FIG. 11 is a schematic block diagram and flowchart illustrating inmore detail the track/stepper of FIG. 7.

[0025] For simplicity in description, identical components are labeledby the same numerals in this application.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] As shown in FIG. 1, a broadband source of electromagneticradiation 20 provides a polychromatic beam with wavelength componentspreferably from 180 to 1000 nm. In one embodiment, source 20 may be axenon lamp. The beam 22 from source 20 is shaped by beam shaping optics24. A portion of the beam 22 is diverted by a beam divider 26 tospectrometer 28 for monitoring intensity variations in beam 22 in orderto normalize the results of detection. Beam 22 is polarized by apolarizer 30 and the polarized beam is directed to sample 32 toilluminate a periodic structure thereon. Preferably the plane ofincidence 34 (see FIG. 1B) of beam 22 is perpendicular to the gratinglines in the periodic structure of sample 32. Radiation originating frombeam 22 that has been modified by sample 32, such as by reflection ortransmission, is collected by collection optics 42 as a collected beam40 and passed through an analyzer 44 which splits the collectedradiation into two rays: an extraordinary ray or e-ray 46 and anordinary ray or o-ray 48. Rays 46 and 48 are detected respectively bytwo spectrometers 52, 54 with outputs delivered to computer 60 alongwith the output of spectrometer 28 for determining one or moreparameters of a periodic structure on sample 32.

[0027] Depending on the orientation of the polarizer 30 and analyzer 44,the e-ray and o-ray contain information concerning two differentradiation parameters. This is illustrated more clearly in reference toFIG. 1B. As shown in FIG. 1B, R_(s) is the reflectance of the Scomponent of beam 40 in the direction normal to the plane of incidenceand R_(p) is the reflectance of the P component of beam 40 within theplane of incidence 34 of beam 22. Assuming that polarizer 30 is orientedat an angle to the plane of incidence 34, where the angle is other than0°, 90°, 180° or 270°, then beam 40 will contain both non-zero Pcomponents and S components. When a plane of polarization is said to beat an angle to the plane of incidence in this application, this anglecan be of any value, including 0 and 180 degrees, unless statedotherwise. For example, beam 33 may have the input polarization alongthe plane 62 at an angle between 0 and 90 degrees to plane 34 in FIG.1B. In such event, depending on the orientation of the plane ofpolarization of analyzer 44, different radiation parameters aremeasured. For example, if the plane of polarization of analyzer 44 isaligned with the plane of incidence 34 or perpendicular to the plane ofincidence, then the components R_(p), R_(s) are measured, and the o-rayand e-ray would yield these two intensities. On the other hand, if theplane of polarization of the analyzer 44 is aligned with the inputpolarization plane 62 of beam 33, or is arranged to be perpendicular toplane 62, then one of the two rays 46, 48 would contain informationconcerning a signal X and the other ray would contain informationconcerning a signal Y (a vector perpendicular to signal X), where X andY are defined as follows:

X=|r _(s) −r _(p)|²

Y=|r _(s) +r _(p)|²

[0028] where r_(s) and r_(p) denote the complex amplitude reflectioncoefficients for S and P polarizations, respectively, while R_(s) andR_(p) are the intensity reflection coefficients.

[0029] If the plane of polarization of polarizer 30 is in the plane ofincidence 34 or perpendicular to it, it is not possible to measure bothR_(p) and R_(s) simultaneously or measure X and Y simultaneously. Thisis true when the grating is perpendicular or parallel to the plane ofincidence 34. In other words, where the grating is not perpendicular orparallel to the plane of incidence 34, it is possible to measure bothR_(p) and R_(s) simultaneously or measure X and Y simultaneouslyirrespective of the plane of polarization of beam 33. It is alsopossible to remove polarizer 30 altogether so that the beam 22illuminating sample 32 is unpolarized. In such event, in order tomeasure R_(s) and R_(p), the plane of polarization of analyzer 44 ispreferably aligned with the plane of incidence 34 or perpendicular toit. Even where the plane of polarization of analyzer 44 is not parallelto the plane of incidence 34 or perpendicular to it, it is possible tomeasure quantities related to R_(s) and R_(p), which would be useful formeasuring the structure.

[0030] Preferably the plane of polarization of the analyzer 44 is at anangle of 0, 45, 90, 135, 180, 225, 270 or 315 degrees from the plane ofincidence 34, since this will optimize the amplitudes of the quantitiesmeasured. Where the plane of polarization of the analyzer 44 is at anangle of 0, 90, 180 or 270 degrees to plane 34, R_(p), R_(s) may bemeasured. Where the plane of polarization of the analyzer 44 is at anangle of 45, 135, 225 or 315 degrees to plane 34, X, Y may be measured.Where the polarizer 30 is oriented at one of the angles of 0, 45, 90,135, 180, 225, 270 or 315 degrees from the. plane of incidence 34, againthe amplitudes of the quantities measured will be optimized.

[0031] After R_(p), R_(s), X, Y have been measured, it is then possibleto derive other radiation parameters therefrom in computer 60, so that amore complete list of quantities that can be obtained from themeasurement are the following:

R_(s), R_(p), R_(s)−R_(p), (R_(s)−R_(p))/(R_(s)+R_(p))

X=|r _(s) −r _(p)|² , Y=|r _(s) +r _(p)|²

X/Y,

X−Y≡−4|r _(s) ||r _(p)|cos Δ

(X−Y)/(X+Y)≡sin 2ψ cos Δ

cos Δ, ${\tan \quad \psi} = {\frac{r_{p}}{r_{s}}}$

[0032] where r_(s) and r_(p) denote the complex amplitude reflectioncoefficients for S and P polarizations respectively, while R_(s) andR_(p) are the intensity reflection coefficients: R_(s)=|r_(s)|²,R_(p)=|r_(p|) ². Δ denotes the phase difference between reflected s- andp-polarized fields. The quantities tan Ψ and cos Δ are ellipsometricparameters known to those in the art.

[0033] The radiation parameter (Rs−Rp)/(Rs+Rp) has better sensitivitythan Rs or Rp alone, since the reflectance difference (Rs−Rp) would tendto cancel out unwanted reflectance from isotropic film structuressurrounding, over or underneath the grating targets when theillumination beam is at normal or near normal incidence. The ratio alsoreduces effects of intensity variations of the illumination beam on themeasurement. By canceling out or reducing unwanted reflectance fromisotropic film structures surrounding, over or underneath the gratingtargets, this feature renders the detection system more robust againststray reflectance around the grating target, and reduces the influenceof thickness variations of films over or under the grating. Since thearea available for the grating target is limited, where the opticalsystem is not exactly aligned properly, some of the radiation from theillumination beam may be reflected from an area outside of the target,so that by using the computer 60 to calculate the reflectancedifference, the measurement is much more robust despite opticalmisalignment errors, and reduces the influence of thickness variationsof films over or under the grating. These variations are caused byprocess variations that introduce error in CD measurement. By measuringdifference in reflectance, these unwanted effects tend to cancel toprovide a more accurate CD measurement. As noted above, the output ofspectrometer 28 may be used to normalize the output of spectrometers 52,54 in order to eliminate the effects of intensity variation from source20.

[0034] Where it is desirable to employ a compact system for detectingperiodic structures, such as where the device is to be integrated withprocessing equipment, the configuration shown in FIG. 1C may beparticularly useful. As shown in FIG. 1C, the incident beam 22 is passedthrough an element 30′ which can be a polarizer, focused by optics 24′to sample 32. The zeroth order diffraction from the periodic structureon sample 32 is collected by focusing optics 24′ and passed through ananalyzer 44′. The functions of the polarizer 30′ and analyzer 44′ aresimilar to those of polarizer 30 and analyzer 44 of FIG. 1A describedabove. For a compact design, polarizer 30′ and analyzer 44′ may be asingle integral unit or are separate elements attached together to forman integrated unit 62 so that the configuration of FIG. 1C isparticularly compact. This allows the apparatus of FIG. 1C to have aparticularly small footprint.

[0035]FIG. 2 is a schematic view of an apparatus 100 for measuring aperiodic structure to illustrate another embodiment of the invention.For simplicity, the computer that processes the output of the twospectrometers 28, 54 has been omitted to simplify the drawing. System100 differs from system 10 of FIG. 1A in that in system 100, either oneor both of the polarizer 30 and analyzer 44 may be rotated whereas thepolarizer and analyzer in system 10 of FIG. 1A are stationary and do notmove. Furthermore, system 100 includes a single spectrometer fordetecting the collected radiation instead of two spectrometers as inFIG. 1A. However, by rotating the polarizer 30 or analyzer 44, two ormore radiation parameters may be detected. The operation of system 100is different from an ellipsometer in that, for at least one measurement,the polarizer 30 and analyzer 44 are stationary and do not move. Thispermits system 100 to measure quantities such as R_(s), R_(p) whereas itwould be extremely difficult for an ellipsometer to measure R_(s), R_(p)accurately.

[0036] System 100 operates under principles similar to that of FIG. 1A.Thus, if the plane of polarization of polarizer 30 is at an angledifferent from 0, 90, 180 and 270 degrees to the plane of incidence ofbeam 22, the plane of polarization of analyzer 44 may be oriented in thesame manner as that described above in reference to FIG. 1B to measureat least one of the parameter values R_(s), R_(p), X, Y. Since only onespectrometer is used instead of two as in FIG. 1A, the quantities aremeasured sequentially rather than simultaneously as in the embodiment ofFIG. 1A. Thus, in reference to FIG. 1B, if the plane of polarization ofanalyzer 44 is aligned with the plane of incidence 34 of beam 22, thenthe spectrometer would detect the radiation parameter R_(p). Thenanalyzer 44 is rotated so that its plane of polarization issubstantially normal to plane 34; at this stationary position ofanalyzer 44, the spectrometer 52 measures the radiation parameter R_(s).The same can be said for the measuring of X, Y, where analyzer 44 isrotated so that its plane of polarization is substantially parallel andnormal to plane 62, where the measurements are conducted sequentiallyrather than simultaneously. In other words, analyzer 44 may besequentially oriented so that its plane of polarization is substantiallyat some of the angles of about 0, 45, 90, 135, 180, 225, 270 or 315degrees to the plane of incidence 34, in order to measure one or more ofthe parameter values R_(s), R_(p), X, Y.

[0037]FIG. 3 is a schematic view of illumination optics 24″ andcollection optics 42″ to illustrate another aspect of the invention. CDmeasurements require a dedicated grating-type target that is printed onthe scribe line between dies on a semiconductor wafer. The areaavailable for such target is typically limited so that the target sizeis typically kept to a minimum, such as around 25 microns square. Even asmall percentage of light reflected from areas outside of the gratingtarget can affect the measurement accuracy. For this reason, arelatively large focusing numerical aperture is employed to keep theenergy of the illumination beam focused on the small target area. Thus,as shown in FIG. 3, the illumination numerical aperture 72 is relativelylarge. A large numerical aperture in the collection optics increases thecomputing time for modeling since the spectrum at more incident angleswould need to be calculated. For this reason, it may be desirable toemploy a small collection numerical aperture, such as aperture 74 shownin FIG. 3, for the collection optics. While FIG. 3 illustrates thecollection optics in transmission mode, the same can be applied toreflection mode, such as those in FIGS. 1A, 1C and 2.

[0038] In another embodiment of the invention, both the apparatus andthe wafer are moved relative to each other. Where the apparatus is movedin a translational motion along a straight line, such motion is easy tocontrol despite the size and the number of is components in theapparatus. Where the wafer is moved in the direction perpendicular tothe motion of the apparatus, the footprint of the entire system can bereduced by making the dimension of the apparatus elongated along thedirection of motion of the wafer, in order to minimize the dimension ofthe apparatus along the direction of its motion, in order to minimizefootprint. This is illustrated in FIG. 4A. As shown in FIG. 4A, anoptics head 102 is mounted onto a translation track 104 and a motor (notshown) causes head 102 to move along the track 104 along direction 106.Track 104 is attached onto supporting base 112, having tracks 115. Thesemiconductor wafer 32′ is placed onto a chuck 114 which is caused byanother motor (not shown) to move along track 115 along direction 116.Where directions 106, 116 are transverse to each other, such asperpendicular to each other, the movement of the optics head 102 andwafer 32′ as described above would enable the optics head 102 to inspectthe entire surface area of sample or wafer 32′. As will be evident fromFIG. 4A, the optics head 102 is elongated in shape, with the elongateddimension substantially along direction 116, the direction of motion ofwafer 32′. This reduces the dimension of optics head 102 in thedirection of motion 106 of the head, thereby reducing the footprintrequired. Thus, as shown in FIG. 4A, the footprint of the entireassembly shown in FIG. 4A has the dimension along direction 106substantially equal to the sum of the dimension of wafer 32′ and opticshead 102. The footprint of the assembly along direction 116 issubstantially twice the dimension of wafer 32′ along such direction.

[0039]FIG. 4B illustrates in perspective view an assembly 120 that issimilar to assembly 110 of FIG. 4A, except that assembly 120, the opticshead contains a radiation source and all collection optics and sensorcomponents, so that it has no provision for an optical fiber leading tothe optics head. In FIG. 4A, on the other hand, the optics head 102 doesnot include a source of radiation, so that an optical channel, such asan optical fiber 118, is employed to supply radiation to optics head102. Track 104 has a passage therein to provide a conduit for theoptical channel 118 to the optics head. In FIG. 4B, the optics head 102′contains a radiation source, so that track 104′ does not need to providethe passage for an optical channel.

[0040]FIG. 4C is a schematic diagram of a semiconductor wafer and ofoptics arranged so that the optical path lengths to all positions of theoptics head remain substantially constant irrespective of the positionof the optics head relative to the wafer to illustrate yet anotherembodiment of the invention. In FIG. 4C, radiation used for inspectionis supplied along an optical channel 118. When the optics head is atposition 102(1) or D, radiation along channel 118 would reach the opticshead 102 by reflection by means of folding mirror 120 (at position120(1) or C) to the optics head. In other words, the optical path lengthwould include the paths BC and CD. In the embodiment shown in FIG. 4C,the wafer is moved along direction 116 or one parallel to the x axis,and the optics head is moved along direction 122 substantially at 45degrees to the x axis. The folding mirror is moved along the y axis at aspeed such that the mirror and the optics head are maintained to bealong a line parallel to the x axis or direction 116. Thus, after theoptics head and the wafer are both moved so that the optics head is nowat position 102(2) or A, and the mirror at position 120(2) or B, theoptics head will receive radiation from channel 118 along a differentoptical path AB than it received when it was at position 102(1). Sincethe line AD is at 45 degrees to direction 116, the optical path lengthAB is equal to the sum of the path lengths BC+DC. If the optical beamalong channel 118 is not collimated, any optical path differencedepending on the positions of the optics head may cause complications inthe measurement process. At a minimum, this may call for a calibrationprocess which may be cumbersome. For this reason, in the embodiment ofFIG. 4C, the optics for directing radiation from channel 118 todifferent positions of the optics head is designed so that the totaloptical path length remains substantially the same irrespective of theposition of the optics head.

[0041] In yet another embodiment, to further reduce the footprint of thesystem, the wafer is rotated while the apparatus for measuring theperiodic structure is moved along substantially a straight line, so thatthe illumination beam from the apparatus scans a spiral path on thewafer. This is shown in FIG. 5. Thus, the optics head 102′ is mountedonto a translation track 104′ as in FIG. 4B and the optics head is movedalong the track along direction 106 by a motor (not shown) as before.The wafer (not shown), however, is situated on top of a support 124which is rotated along direction 126 by a motor (not shown). In thismanner, the optics head 102′ would have access to any location on thewafer for taking measurement.

[0042]FIG. 6 is a block diagram of an optical arrangement 140illustrating a practical implementation for the optics head 102 of FIGS.4A, 4C. As shown in FIG. 6, the optics head 102 a contains an opticalarrangement similar to that in FIG. 1A, where the analyzer 44 splits thecollected beam 40 into an o-ray directed to spectrometer 54 and an e-raydirected to spectrometer 52 through a prism 141, which compensates forthe dispersion caused by the analyzer 44 for the deviated beam. Thelight from light source is conveyed through an optical fiber 20′ andreflected by a collimating mirror 142 to polarizer 30 and apodizer 144in a collimated light piston to optics head 102 a. In optics head 102 a,the light is reflected by mirror 146 to the beam shaping optics 24, 42comprising two focus/collimating mirrors to sample 32. The samefocus/collimating mirrors 24/42 collect light reflected by sample 32 toanalyzer 44. As will be noted from FIG. 6, collimating or focusingmirrors are used for directing and focusing radiation from the lightsource cross the sample 32 and for collecting light reflected by thesample towards the analyzer. By using mirrors for such purposes,chromatic aberration is reduced, especially where the detection employsradiation over a wide range of wavelengths, such as 150 nanometers to1,000 nanometers.

[0043]FIG. 7 is a schematic view of another alternative construction 102b of the optics head in FIGS. 4A, 4C. As shown on FIG. 7, optics head102 b includes an optical fiber 20′ conveying radiation from a radiationsource (not shown) through polarizer 30 and apodizer 144 and focused bymirror 152 to sample 32. Sample 32 is moved by a stage 148 along onedirection (x) or along two transverse (x/y) directions. Radiationreflected by sample 32 is collected by collection mirror 154 and focusedto an analyzer 44. The radiation that passes the analyzer is conveyed toa spectrometer (not shown) by fiber 156. Analyzer 44 is rotated todesirable positions as described above in reference to FIG. 2.

[0044]FIG. 8 is a schematic view of the different components in theoptics head 102′ to illustrate in more detail the optics head of FIGS.4B, 5. As shown in FIG. 8, optics head 102′ includes radiation source 20which supplies a beam of radiation that passes polarizer 30 and isreflected by a beam splitter 162 and focused by a broadband objective164 to wafer 32. The reflected radiation from the wafer is collected byobjective 164, passed through beam splitter 162 and reflected by mirror166 to analyzer 44 to spectrometer 54. Preferably the analyzer 44 isrotated so that its plane of polarization and the plane of polarizationof the polarizer 30 are as described above in reference to FIG. 2.

[0045] The process for deriving one or more parameters of the periodicstructure using a computer such as computer 60 will now be described inreference to FIG. 9 which is a flowchart illustrating the operation ofapparatuses of FIGS. 1A,1B and 2. First, one obtains the possible rangeof values of the one or more parameters of the periodic structure, suchas critical dimension (CD), height (H), sidewall angle (SWA) and otherparameters (block 202). A theoretical model is then constructed (block204) and a multi-dimensional library is constructed (block 206).

[0046] One of the apparatuses of FIG. 1A, 1C and 2 is then utilized toobtain a measurement of the zeroth order diffraction of the periodicstructure in the manner described above (block 208), where suchdiffraction is detected by a single or dual channel spectrometer asdescribed above (block 210). As described above, the quantities R_(s),R_(p) and X, Y may be measured directly using the apparatuses of FIGS.1A, 1C and 2. From these four quantities, other radiation parameters maybe derived in the manner described above (block 212). Themulti-dimensional library 206 constructed is then utilized andcomparison is made between spectra in the library and the measured datafrom block 212. The parameters of the model are varied (block 206) inorder to minimize the error between the spectra corresponding to thepredicted values of the parameters of the periodic structure accordingto the model, and the measured spectra from block 212 in order to arriveat the actual values of the parameters of the periodic structure, suchas CD, height and sidewall angle (diamond 214).

[0047] Alternatively, instead of using a multi-dimensional library, anon-linear optimization (e.g. non-linear regression) algorithm may beemployed instead. For this purpose, the initial or seed values of theparameters of the periodic structure are obtained (block 222). Anoptimization model is constructed (block 224) and a non-linearregression algorithm (block 226) is employed with feedback (diamond 214)to again minimize the error between the spectra of radiation parameterscorresponding to the predicted values of the parameters of the periodicstructure and the measured spectra from block 212 in order to arrive atthe actual values of parameters such as CD, height and sidewall angle(block 216).

[0048]FIG. 10 is a block diagram of an integrated diffraction-basedmetrology system, a, photolithographic track/stepper and an etcher toillustrate another aspect of the invention. A layer of material such asphotoresist is formed on the surface of a semiconductor wafer by meansof track/stepper 350, where the photoresist forms a grating structure onthe wafer. One or more of the CD, H, SWA and/or other parameters of thegrating structure are then measured using systems 10, 80, 100 of FIG.1A, 1C and 2, and one or more of the above-described techniques may beemployed if desired to find the value(s) of the one or more parametersof the photoresist pattern. Such value(s) from the computer 60 are thenfed back to the track/stepper 350, where such information may be used toalter the lithographic process in track/stepper 350 to correct anyerrors. In semiconductor processing, after a layer of photoresist hasbeen formed on the wafer, an etching process may be performed, such asby means of etcher 360. The layer of photoresist is then removed in amanner known in the art and the resulting grating structure made ofsemiconductor material on the wafer may again be measured if desiredusing system 10, 80 or 100. The value(s) measured using any one or moreof the above-described techniques may be supplied to the etcher foraltering any one of the etching parameters in order to correct anyerrors that have been found using system 10, 80 or 100. Of course theresults obtained by one or more of the above described techniques insystem 10, 80 or 100 may be used in both the track/stepper and theetcher, or in either the track/stepper or the etcher but not both. Thetrack/stepper 350 and/or etcher 360 may form an integrated single toolwith the system 10, 80 or 100 for finding the one or more parameters ofa periodic structure, or may be separate instruments from it.

[0049]FIG. 11 is a schematic view of the track/stepper 350 and anassociated flowchart illustrating a process for semiconductor waferprocessing to illustrate in more detail the points of integration of theprocessing process with the detection of profiles of periodic structuresand associated films to illustrate in more detail a part of the processin FIG. 10. As shown in FIG. 11, a semiconductor wafer 352 may be loadedfrom a cassette loader 354 to several stations labeled “prime,” “coat,”“soft bake,” “EBR” etc. Then the wafer 352 is delivered by a stepperinterface 356 to exposure tool 358. The different processes at the fourlocations mentioned above are set forth below:

[0050] At the location “Prime”, the wafer undergoes chemical treatmentbefore a layer of photoresist is spun on it, so that the photoresistlayer can stick to wafer. At the location “Coat”, a layer of photoresistcoating is spun onto the wafer. At “Soft bake”, the layer of resist isbaked to remove chemical solvent from the resist. At “EBR” which standsfor “edge-bead removal”, a solvent nozzle or laser is used to removeexcess photoresist from the edge of wafer.

[0051] After the wafer has been exposed to radiation by tool 358, thewafer then undergoes four additional processes: “PEB,” “PEB chill,”“Develop,” and “Hard bake.”At “PEB or post exposure bake”, the wafer isbaked to reduce standing-wave effect from the exposure tool. Then it iscooled at “PEB chill”. The wafer is then washed with reagent to developthe photoresist, so that un-exposed (negative) or exposed (positive)photoresist is removed. The wafer then is baked at “Hard bake” tostabilize the photoresist pattern. It will be noted that all of thecomponents of device 350 of FIG. 8 except for the “exposure tool” 358 isknown as the “Track” (also called cluster).

[0052] After these latter four processes have been completed, the wafer352 is then returned to the cassette loader 354 and this completes theprocessing involving the stepper 350. The detection system 10, 80 or 100may be applied at arrow 362 to measure the parameters of the periodicstructure and associated film(s). Thus, such parameters may be measuredafter “hard bake.”

[0053] While the invention has been described above by reference tovarious embodiments, it will be understood that changes andmodifications may be made without departing from the scope of theinvention, which is to be defined only by the appended claims and theirequivalents. All references referred to herein are incorporated byreference in their entirety.

What is claimed is:
 1. A method for measuring one or more parameters of a periodic structure, comprising: directing a polychromatic beam of electromagnetic radiation to the structure; collecting radiation from the beam after it has been modified by the structure; dividing the collected radiation into two collected rays having different polarization states; detecting the two rays to provide two outputs; and deriving the one or more parameters from the two outputs.
 2. The method of claim 1, wherein the dividing divides the collected radiation into an ordinary ray and an extraordinary ray, said two rays having substantially orthogonal polarizations.
 3. The method of claim 1, wherein the directing includes passing the collected radiation through an optical element having a plane of polarization at an angle different from 0, 90, 180 and 270 degrees to the plane of incidence.
 4. The method of claim 1, wherein the dividing includes passing the collected radiation through an optical element having a plane of polarization at an angle of about 0, 45, 90, 135, 180, 225, 270 and 315 degrees to the plane of incidence.
 5. The method of claim 1, wherein the directing directs an unpolarized beam to the structure, and wherein the dividing includes passing the collected radiation through an optical element having a plane of polarization at an angle of about 0, 90, 180 and 270 degrees to the plane of incidence.
 6. An apparatus for measuring one or more parameters of a periodic structure, comprising: an instrument directing a polychromatic beam of electromagnetic radiation to the structure; optics collecting radiation from the beam after it has been modified by the structure; a device dividing the collected radiation into two collected rays having different polarization states; detectors detecting the two rays to provide two outputs; and a processor deriving the one or more parameters from the two outputs.
 7. The apparatus of claim 6, wherein the device divides the collected radiation into an ordinary ray and an extraordinary ray, said two rays having substantially orthogonal polarizations.
 8. The apparatus of claim 6, wherein the instrument includes an optical element having a plane of polarization at a non-zero angle to the plane of incidence, wherein said plane of polarization is not perpendicular to the plane of incidence.
 9. The apparatus of claim 6, wherein the device includes an optical element having a plane of polarization at an angle to the plane of incidence, where the angle is about 0, 45, 90, 135, 180, 225, 270 and 315 degrees.
 10. The apparatus of claim 6, wherein the instrument directs an unpolarized beam to the structure, and wherein the device includes an optical element passing the collected radiation, said optical element having a plane of polarization at an angle of about 0, 90, 180 and 270 degrees to the plane of incidence.
 11. The apparatus of claim 4, wherein each of said source and said optics has a numerical aperture, and wherein the numerical aperture of the optics is smaller than that of the source.
 12. A method for measuring one or more parameters of a periodic structure, comprising: (a) directing a polychromatic beam of electromagnetic radiation to the structure in a plane of incidence; (b) collecting radiation from the beam after it has been modified by the structure; (c) passing the collected radiation through a first polarizing element having a polarization plane at a first angle to the plane of incidence; (d) detecting the collected radiation passing through the element to provide an output; (e) altering the first angle between the two planes to a different value and repeating (a), (b), (c) and (d), wherein said different value remains substantially stationary when (a), (b), (c) and (d) are repeated to provide at least an additional output; and (f) deriving the one or more parameters from the outputs.
 13. The method of claim 12, wherein said different stationary value of the angle is one of 0, 45, 90, 135, 180, 225, 270 and 315 degrees.
 14. The method of claim 12, wherein said directing includes passing radiation through a second polarizing element having a polarization plane at a second angle to the plane of incidence, said second angle having a value different from 0, 90, 180 and 270 degrees.
 15. The method of claim 14, wherein said polarization planes of the two elements are substantially parallel or perpendicular to each other.
 16. An apparatus for measuring one or more parameters of a periodic structure, comprising: a source directing a polychromatic beam of electromagnetic radiation to the structure in a plane of incidence; optics collecting radiation from the beam after it has been modified by the structure; a first polarizing element having a polarization plane at a first angle to the plane of incidence passing the collected radiation; a detector detecting the collected radiation that has passed through the element to provide an output; an instrument rotating the first element relative to the plane of incidence to alter the value(s) of the first angle to one or more different value(s) that remain substantially stationary when said detector is detecting the collected radiation, so that the detector provides at least one output before and after the first angle is altered; and a processor deriving the one or more parameters from the outputs.
 17. The apparatus of claim 16, wherein said different value of the first angle is one of 0, 45, 90, 135, 180, 225, 270 and 315 degrees.
 18. The apparatus of claim 16, said source including a second polarizing element passing radiation to provide said beam, said second element having a polarization plane at a second angle to the plane of incidence, said instrument rotating one or more of the two elements relative to the plane of incidence to alter the value(s) of the first and/or the second angle to one or more different value(s) that remain substantially stationary when said detector is detecting the collected radiation.
 19. The apparatus of claim 18, wherein said different value(s) of the first and/or second angle are one of 0, 45, 90, 135, 180, 225, 270 and 315 degrees.
 20. The apparatus of claim 16, wherein each of said source and said optics has a numerical aperture, and wherein the numerical aperture of the optics is smaller than that of the source.
 21. The apparatus of claim 16, wherein said source includes a second polarizing element having a polarization plane at an angle to the plane of incidence, said second element passing radiation to provide said beam, said angle having a value different from 0, 90, 180 and 270 degrees.
 22. The apparatus of claim 21, wherein said polarization planes of the two elements are substantially parallel or perpendicular to each other.
 23. An apparatus for measuring one or more parameters of a periodic structure, comprising: an optical device including a first element directing a polychromatic beam of electromagnetic radiation to the structure in a plane of incidence and a second optical element passing radiation from the beam after it has been modified by the structure, said two elements attached together to form an integrated unit or being an integral unit; said second element having a plane of polarization; and at least one detector detecting the collected radiation that has passed through the second element to provide at least one output.
 24. The apparatus of claim 23, said plane of polarization is at an angle to the plane of incidence, said angle having a value different from 0, 90, 180, 270 degrees.
 25. The apparatus of claim 24, wherein said plane of polarization is at an angle of about 0, 45, 90, 135, 180, 225, 270 or 315 degrees to the plane of incidence.
 26. The apparatus of claim 23, wherein the second element divides the radiation from the beam after it has been modified by the structure into an ordinary ray and an extraordinary ray, said two rays having substantially orthogonal polarizations, said apparatus further comprising two detectors, each of the two detectors detecting a corresponding one of the two rays.
 27. The apparatus of claim 23, wherein each of said two elements has a numerical aperture, and wherein the numerical aperture of the second element is smaller than that of the first.
 28. The apparatus of claim 23, further comprising a processor deriving the one or more parameters from the output.
 29. An apparatus for inspecting a sample having a structure thereon, comprising: (a) a detection system including: a device directing a polychromatic beam of electromagnetic radiation to the structure; optics collecting radiation from the beam after it has been modified by the structure; and at least one detector detecting the collected radiation to provide at least one output; (b) a first instrument causing translational motion of the sample in a first direction; and (c) a second instrument causing translational motion between the first instrument and the system in a second direction transverse to the first direction.
 30. The apparatus of claim 29, said system further including a radiation source.
 31. The apparatus of claim 29, said system further including a conduit carrying a collimated beam of radiation.
 32. The apparatus of claim 29, further including an optical arrangement directing an incoming radiation beam to the detection system along different optical paths when relative motion is caused between the system and the sample, so that the different optical paths have substantially the same optical path length.
 33. The apparatus of claim 29, said arrangement including a radiation reflective element that moves together with the second instrument reflecting radiation towards the device along optical paths that are substantially at 45 degrees to a trajectory of the device when moved by the two instruments.
 34. An apparatus for inspecting a sample having a structure thereon, comprising: (a) a detection system including: a device directing a polychromatic beam of electromagnetic radiation to the structure; optics collecting radiation from the beam after it has been modified by the structure; and at least one detector detecting the collected radiation to provide at least one output; (b) an instrument causing first motion of the sample, and second motion between the first instrument and the system, wherein one of the two motions is translational and the remaining motion is rotational.
 35. The apparatus of claim 34, said system further including a radiation source.
 36. The apparatus of claim 34, said system further including a conduit carrying a collimated beam of radiation.
 37. The apparatus of claim 34, further including an optical arrangement directing an incoming radiation beam to the detection system along different optical paths when relative motion is caused between the system and the sample, so that the different optical paths have substantially the same optical path length.
 38. The apparatus of claim 34, said arrangement including a radiation reflective element that moves together with the second instrument reflecting radiation towards the device along optical paths that are substantially at 45 degrees to a trajectory of the device when moved by the two instruments.
 39. An integrated processing and detection apparatus for processing a sample having structures thereon, comprising: (a) a detection system for finding one or more parameters of a structure, wherein the system detects the structure by directing a polychromatic beam of electromagnetic radiation to the structure, collecting radiation from the beam after it has been modified by the structure; said system including: a device dividing the collected radiation into two collected rays having different polarization states; detectors detecting the two rays to provide two outputs; and a processor deriving the one or more parameters from the two outputs; and (b) a processing system processing the sample, said processing system responsive to said one or more parameters for adjusting a processing parameter.
 40. The apparatus of claim 39, said detection system further including a radiation source that provides the polychromatic beam.
 41. The apparatus of claim 39, further including a conduit for transmitting radiation to said detection system.
 42. The apparatus of claim 41, said conduit including an optical fiber.
 43. The apparatus of claim 39, further including an instrument causing relative motion between the detection system and the sample in order to detect an area of the sample, an optical arrangement directing an incoming radiation beam to the detection system along different optical paths when relative motion is caused between the system and the sample, so that the different optical paths have substantially the same optical path length.
 44. The apparatus of claim 39, said detection system including one or more reflective optical elements.
 45. An integrated processing and detection apparatus for processing a sample having structures thereon, comprising: (a) a detection system for finding one or more parameters of a structure, wherein the system detects the structure by directing a polychromatic beam of electromagnetic radiation to the structure in a plane of incidence, collecting radiation from the beam after it has been modified by the structure; said detection system including: a first polarizing element having a polarization plane at a first angle to the plane of incidence passing the collected radiation; a detector detecting the collected radiation that has passed through the element to provide an output; an instrument rotating the first element relative to the plane of incidence to alter the value(s) of the first angle to one or more different value(s) that remain substantially stationary when said detector is detecting the collected radiation, so that the detector provides at least one output before and after the first angle is altered; and a processor deriving the one or more parameters from the outputs. (b) a processing system processing the sample, said processing system responsive to said one or more parameters for adjusting a processing parameter.
 46. The apparatus of claim 45, said detection system further including a radiation source that provides the polychromatic beam.
 47. The apparatus of claim 45, further including a conduit for transmitting radiation to said detection system.
 48. The apparatus of claim 47, said conduit including an optical fiber.
 49. The apparatus of claim 45, said system further including a second instrument causing relative motion between the detection system and the sample in order to detect an area of the sample, said system further including an optical arrangement directing an incoming radiation beam to the detection system along different optical paths when relative motion is caused between the system and the sample, so that the different optical paths have substantially the same optical path length.
 50. The apparatus of claim 45, said detection system including one or more reflective optical elements.
 51. An integrated processing and detection apparatus for processing a sample having structures thereon, comprising: (a) a detection system including: a device directing a polychromatic beam of electromagnetic radiation to the structure; optics collecting radiation from the beam after it has been modified by the structure; and at least one detector detecting the collected radiation to provide at least one output; (b) a first instrument causing motion of the sample; (c) a second instrument causing relative motion between the first instrument and the system so that the beam has access to any location of the sample; and (d) a processing system processing the sample, said processing system responsive to said at least one output for adjusting a processing parameter.
 52. The apparatus of claim 51, said detection system further including a radiation source that provides the polychromatic beam.
 53. The apparatus of claim 51, further including a conduit carrying a collimated beam of radiation to the detection system.
 54. The apparatus of claim 51, further including an optical arrangement directing an incoming radiation beam to the detection system along different optical paths when relative motion is caused between the system and the sample, so that the different optical paths have substantially the same optical path length.
 55. The apparatus of claim 51, said two instruments causing translational motion that are substantially perpendicular to each other, said arrangement including a radiation reflective element that moves together with the second instrument reflecting radiation towards the device along optical paths that are substantially at 45 degrees to a trajectory of the device when moved by the two instruments. 